1. Field of the Invention
This invention relates to an extremely thin film of polyethylene and more particularly, it is concerned with a transparent and extremely thin film of ultrahigh molecular weight polyethylene excellent in tensile modulus as well as breaking strength. The term "extremely thin" will hereinafter be referred to as "superthin" in this specification.
2. Description of the Prior Art
The general features of polyethylene consist in being excellent in chemical resistance, electrical insulation and moldability and not brittle even at low temperatures and films obtained therefrom have widely been used as various packing materials and electrical insulating materials. For the electrical insulating materials, e.g. condensers, above all, thinner and higher strength films have lately been required. In the case of making thinner a polyethylene film having an ordinary molecular weight, for example, less than 500,000, however, the thickness obtained is limited to approximately 10 .mu.m and when the polyethylene film is further made thinner than this thickness, it is not resistant to the tension during stretching and often broken. On the other hand, ultrahigh molecular weight polyethylene is expected to have a higher strength and higher elasticity even in the form of a superthin film, but in actual fact, it has a worse stretchability because of more remarkable interlacement of the molecular chains than the ordinary molecular weight polyethylene. In order to improve this stretchability, for example, there has been proposed a method comprising dissolving ultrahigh molecular weight polyethylene in paraffin oil, forming a gel film from this solution, and then heating and stretching the gel containing the solvent or dry gel from which the solvent has been extracted with a volatile solvent, as described in Japanese Patent Application OPI (Kokai) No. 5228/1983.
However, the gel film of ultrahigh molecular weight polyethylene has a continuous and porous network structure swelled highly with a non-volatile solvent, so stretching it in two directions as it is results in that a high orientation stretching is impossible, the meshes of the network is enlarged to cause a tendency of breakage and a high ratio stretching is so difficult that making the film thinner is unfavourably carried out. When the gel film of ultrahigh molecular weight polyethylene containing the solvent is dried by substitution thereof with a volatile solvent, the porous network structure in the gel film is not capable of self-supporting and shrinks to be compacted. Uniform evaporation of the solvent in the gel film is difficult, the dried film bends and a high ratio stretching is impossible due to shrinking and compacting in the production of a superthin film.